24 research outputs found
Dynamical effects in heavy-ion interactions at bombarding energies near the Coulomb barrier: A study for the 16O + 120Sn system
Abstract We investigate the magnitude of the effective interactions for elastic and inelastic processes at bombarding energies in the vicinity of the nominal Coulomb barrier. The relevance of higher-order inelastic and transfer processes in defining the strength of the effective couplings is explored in a reaction with a superfluid target, 16 O on 120 Sn, at center-of-mass energies of 46, 50 and 54 MeV. Significant dynamical effects are found and these appear to be more pronounced in the off-diagonal matrix elements than in the diagonal ones. Theoretical arguments related to the data are presented
One- and two-proton transfer reactions with vibrational Nuclei
We extend a semiclassical model of transfer reactions to the case in which
one of the collision partners is a vibrational nucleus. The model is applied to
one- and two-proton stripping reactions in the 37Cl + 98Mo system, for which a
rapid transition from normal to anomalous slope in the two proton transfer
reaction at energies around the Coulomb barrier is experimentally observed.
This behavior is satisfactorily reproduced by the present extension of the
model.Comment: LaTeX, 10 pages, 1 figure (PostScript
No evidence of break-up effects on the fusion of 9Be with medium-light nuclei
AbstractFusion cross sections were measured for the 9Be+27Al and 19F+9Be, 12C systems, at energies above the Coulomb barrier, in order to investigate the possible effect of fusion hindrance due to the break-up of the weakly bound nuclei. Comparisons with one-dimensional barrier penetration models and with other similar systems, where no break-up is expected to occur, show no evidence of fusion hindrance
Bootstrap method for constructing covariance matrices of optical-model parameters in the study of the threshold anomaly
The parameters of optical-model potentials are usually obtained by adjusting theoretical calculations to the corresponding experimental elastic-scattering data. It has been observed that the use of conventional covariance matrices for the evaluation of the uncertainties of the parameters obtained in this way, leads in general to unrealistically small values. This underestimate may be caused by either, an incorrect use of the statistical recipes, or by the lack of a systematic study of the robustness of the uncertainty values against the inclusion or exclusion of experimental data points within a given data set. In the present contribution we explore both factors. Regarding the first aspect we use a re-normalization for χ2, similar to the one proposed by R.T. Birge. In the second case we use the Bootstrap method to create synthetic sets based on all the available experimental data in order to derive an effective covariance matrix. These procedures were applied to the re-analysis of elastic-scattering data for several heavy-ion systems at energies close to the Coulomb barrie
Be+Sn scattering at near-barrier energies within a four body model
Cross sections for elastic and inelastic scattering of the weakly-bound
Be nucleus on a Sn target have been measured at seven bombarding
energies around and above the Coulomb barrier. The elastic angular
distributions are analyzed with a four-body continuum-discretized
coupled-channels (CDCC) calculation, which considers Be as a three-body
projectile ( + + n). An optical model analysis using the S\~ao
Paulo potential is also shown for comparison. The CDCC analysis shows that the
coupling to the continuum part of the spectrum is important for the agreement
with experimental data even at energies around the Coulomb barrier, suggesting
that breakup is an important process at low energies. At the highest incident
energies, two inelastic peaks are observed at 1.19(5) and 2.41(5) MeV.
Coupled-channels (CC) calculations using a rotational model confirm that the
first inelastic peak corresponds to the excitation of the 2 state in
Sn, while the second one likely corresponds to the excitation of the
3 state.Comment: 11 pages, 9 figures. Accepted as PR
Comprehensive study of reaction mechanisms for the Be9+Sm144 system at near- and sub-barrier energies
The delayed x-ray detection technique was used to measure complete and incomplete fusion cross sections for the reaction at sub- and near-barrier energies. Elastic and inelastic scattering for this system were also measured. Reaction cross sections were derived and the transfer cross sections of one neutron were calculated. The suppression of complete fusion above the barrier, of the order of 10%, is attributed to breakup and is considerably smaller than the value of 30% found for the system
Coupled-channels analysis of the O+Pb fusion barrier distribution
Analyses using simplified coupled-channels models have been unable to
describe the shape of the previously measured fusion barrier distribution for
the doubly magic O+Pb system. This problem was investigated by
re-measuring the fission excitation function for O+Pb with
improved accuracy and performing more exact coupled-channels calculations,
avoiding the constant-coupling and first-order coupling approximations often
used in simplified analyses. Couplings to the single- and 2-phonon states of
Pb, correctly taking into account the excitation energy and the phonon
character of these states, particle transfers, and the effects of varying the
diffuseness of the nuclear potential, were all explored. However, in contrast
to other recent analyses of precise fusion data, no satisfactory simultaneous
description of the shape of the experimental barrier distribution and the
fusion cross-sections for O+Pb was obtained.Comment: RevTex, 29 pages, 7 postscript figures, to appear in PR
Effect of the break-up on the fusion and elastic scattering of weakly bound projectiles on Zn
We study the behavior of the fusion, break-up, reaction and elastic
scattering of different projectiles on Zn, at near and above barrier
energies. We present fusion and elastic scattering data with the tightly bound
O and the stable weakly bound Li, Li and Be
projectiles. The data were analyzed by coupled channel calculations. The total
fusion cross sections for these systems are not affected by the break-up
process at energies above the barrier. The elastic (non-capture) break-up cross
section is important at energies close and above the Coulomb barrier and
increases the reaction cross sections. In addition we also show that the
break-up process at near and sub-barrier energies is responsible for the
vanishing of the usual threshold anomaly of the optical potential and give rise
to a new type of anomaly.Comment: 24 pages, 10 figure